(1) Field of the Invention
The invention relates to an internal broaching tool for internal broaching of internal tooth systems.
(2) Description of the Related Art
Internal broaching tools for internal broaching of profiles are known from DIN 1415 (1973 edition), Sheet 1, Page 2.
These internal broaching tools have a shaft, a tooth set part, and an end piece. For internal broaching, the shaft of the internal broaching tool is clamped into the shaft holder of a broaching machine. The profiles to be produced are broached in that the broaching tool, clamped in place as described, is pulled through a work piece that is also disposed in the broaching machine.
After completion of the broaching process and removal of the broached work piece, the end piece of the broaching tool is grasped by an end piece holder of the broaching machine and transported back to its starting position.
The tooth set part of the broaching tool has broaching teeth disposed in multiple rows, one behind the other, counter to the broaching direction.
These broaching teeth are equipped with profile root blades, for cutting the profile root, on the one hand, and furthermore with profile flank blades, for cutting the profile flanks, on the other hand.
The profile root blades are also referred to as main blades, since they perform the main cutting effort, and the profile flank blades are referred to as secondary blades.
The profile root blades of the broaching teeth assigned to one another are disposed one behind the other and have a depth gradation, i.e. an increase in diameter, counter to the broaching direction.
This brings about the result that all the broaching teeth that serve for cutting a profile cut a chip from the profile root, one behind the other.
The profile flank blades, which serve for cutting the flanks of the profile, have a profile gradient counter to the broaching direction, whereby the profile flank blades of the trailing broaching tooth lie laterally free with reference to the profile flank blades of the leading broaching tooth, i.e. are configured to be “narrower,” so that the trailing broaching tooth always cuts only in the region provided by means of the depth gradation (increase in diameter). In this way, jamming of the broaching teeth in the region of the profile flanks during the broaching process can be prevented.
As a result of the great stresses on the broaching tool, a displacement of the axis of the tool can come about during the broaching process, and as a result, the broaching teeth that remove material, one after the other, have a different center position with regard to the work piece to be broached.
This center deviation can now have a torsion error superimposed on it in the case of helical broaching, as a result of the additional high rotational forces that occur during helical broaching, and as a result, not only the profile precision but also the surface quality of the profile flanks are impaired.
However, since great profile shape and flank precision is required in the case of gear wheels with internal tooth systems, having a slanted tooth system, in order to guarantee wear-free, precise, and quiet running, it is usual in the state of the art to additionally provide a calibration region after the tooth set part equipped with broaching teeth, counter to the broaching direction.
This calibration region on the broaching tool consists of multiple broaching teeth disposed one behind the other, having the same height, which teeth demonstrate tooth thicknesses that become greater counter to the broaching direction.
These calibration broaching teeth cut a chip over the entire height of the profile flank, in each instance, the chip thickness of which generally amounts to 10 to 20 μm per tooth. These calibration broaching teeth are provided with a clearance angle on their profile flank blades.
Good profile shape precision and very good surface quality are achieved by means of calibration broaching.
By means of the internal broaching tools used in the state of the art, having a calibration part, a system-related broaching force interruption occurs when switching from depth-graduated broaching to full-shape calibration, which interruption leads, particularly in the case of helical broaching, to stress relief of the main cutting force that acts counter to the broaching direction, and thus to a reduction in the torsion stress, and as a result, the relative twist between the work piece and the broaching tool changes. This rebound can now bring about the result that the full-shape calibration region is not correctly introduced into the profiles that have already been broached with depth gradation, so that in the case of incorrect “introduction” of the calibration teeth, which possess sharp blades at both tooth flanks, the profile flanks are then cut on one side, so that the profile is then not calibrated on both profile flanks, as was actually intended.
This partial “rebound” must now be taken into consideration as early as in the design of the broaching tool, i.e. the placement of the broaching teeth of the calibration region.
An incorrect “offset” placement of the calibration broaching teeth with regard to the depth-graduated broaching teeth has the result that the entire broaching tool is unusable.
For this reason, an attempt was already made, by means of a solution previously described in EP 0739674 A1, to eliminate the aforementioned disadvantages in the case of internal broaching tools, in that the broaching teeth disposed one behind the other and assigned to one another possess profile flank blades over their full profile height, which blades have a profile pitch that is small as compared with the increase in diameter of the profile root blades, so that in the case of this solution, the broaching teeth become “thicker.”
In the case of this solution, an attempt is made to influence the cutting force in such a manner that a constant torsion force is maintained during the entire broaching process. Relaxation and “rebound” of the torsion twist take place only once the entire broaching process has taken place, and can lead to profile defects.
In order to achieve the desired broaching result by means of this solution, extensive experiments are required. Nevertheless, any variation in work piece material strength characteristics will influence the broaching result and can lead to undesirable profile defects even then.
On the one hand, the complicated and very difficult production is a disadvantage of this solution, since the profile pitches of 1 to 3 μm per tooth can only be measured in total in the machine (for example over 20 teeth).
On the other hand, in the case of this solution presented in EP 0739674 A1, one has no possibility of “correcting” the broaching tool once it has been produced, i.e. for example of changing the tool, if the parameters of the tooth system required by the customer cannot be fulfilled with the broaching tool being used, in such a manner that the customer's wishes can be met with the lowest possible expenditure of costs, so that even in the case of this solution, a new internal broaching tool must always be produced, in time-consuming and cost-intensive manner.
This solution previously described in EP 0739674 A1 was optimized by means of the solution disclosed in EP 1 160 040 A1, whereby the new, optimized solution also, once again, demonstrates the disadvantages explained in connection with EP 0739674 A1.
Furthermore, a broaching tool having a plurality of blade sections having at least three and maximally six teeth, in each instance, in the axis direction, is known from EP 1 184 118 B1, which sections are supposed to avoid an axial displacement, i.e. “untrue running” of the broaching tool during the production of straight profiles. For this purpose, the individual blade sections are separated from one another by means of re-centering guides, which are structured in cylinder shape and have a diameter equal to the diameter of the entry guide, whereby these re-centering guides serve for guidance, on the one hand, and furthermore simultaneously act as a relaxation part between the individual cutting sequences of the internal broaching tool.
The production of such re-centering guides is very complicated and cost-intensive, and nevertheless has the disadvantage that a broaching force interruption in connection with a constant profile gradient leads to an impermissible profile defect, so that the aforementioned solution cannot be used for helical broaching.
A segmented internal broaching tool having multiple guide and relaxation regions disposed between individual cutting sequences was already known from U.S. Pat. No. 1,197,132.
This arrangement of multiple guide and relaxation regions between individual cutting sequences of a segmented internal broaching tool previously described in U.S. Pat. No. 1,197,132 can be produced in significantly simpler manner, but nevertheless also has the disadvantage already explained above, that when this solution is used, a broaching force interruption occurs, which, in connection with a constant profile gradient, leads to an impermissible profile defect, so that this solution also cannot be used for helical broaching.
In EP 1 317 982 B1, another solution is furthermore disclosed, which further develops the solution already previously described in EP 0739674 A1 and which can be used not only on drawing broaching machines but also on lift-table broaching machines, which are increasingly coming into use. In the case of this internal broaching tool, one of the two profile flanks is finish-machined by means of the roughing part, on the entire work piece, to such an extent that this flank no longer has to be reworked by the calibration region.
This flank, which now already has the final shape, functions as a guiding flank during calibration.
For this reason, the blades on the calibration region are disposed on the calibration teeth only on one side in this solution.
All the calibration teeth have a pressure flank without clearance angle in the same direction.
These pressure flanks lay themselves against the guiding flanks of the work piece, which are already (finally) finished, during calibration.
The flanks of the calibration teeth that lie opposite the guiding flanks are blades provided with a clearance angle.
These blades bring about re-calibration of the pre-finished profile flanks that lie opposite the guiding flanks.
However, aside from the disadvantages that occur analogous to the solution according to EP 0739674 A1, another disadvantage results from this, which consists in that during the broaching process, by means of the solution presented in EP 1 317 982 B1, because of the one-sided smoothing machining, no flank shape corrections (for example corrections of the height convexity to improve the running properties of the tooth system) can be undertaken any longer, since flank shape corrections must always take place on both sides.
Furthermore, a broaching tool is known from JP 61214914 A, in which broaching teeth having blades of the profile root blades disposed exclusively on the outer radii (for pre-broaching/rough broaching) are offset on the face side, after the blades, in each instance, in such a manner that as a result, a guidance region is integrated into each broaching cutting tooth, whereby the “rear” part of each broaching cutting tooth forms this additional guidance region.
In the case of this arrangement disclosed in JP 61214914 A, the broaching teeth are exclusively broaching teeth having profile root blades (these blades, as has already been mentioned, are always disposed (on the tooth head) on the outer radii).
These blades, disposed on the outer radius, are ground to form an undercut (offset) (according to JP 61214914 A), and thereby result in “a continuous tooth” having a front blade part and a rear guidance region.
In the case of this solution, only the upper flanks necessarily guide (in the rear guidance region), so that at best, “untrue running” of the broaching tool can be reduced during pre-broaching.